Update README.md

Author: mrts

README.md

@@ -36,7 +36,7 @@ Clone the repository, mount its directory as a volume into
 Clone the repository, build and push the image to your Docker repository,
 for example for Google Private Repository do the following:
 
-    docker build --tag=eu.gcr.io/your-project/postgres-multi-db
+    docker build --tag=eu.gcr.io/your-project/postgres-multi-db .
     gcloud docker -- push eu.gcr.io/your-project/postgres-multi-db
 
 You still need to pass the `POSTGRES_MULTIPLE_DATABASES` environment variable